The present invention relates to a process for preparing polyolefin resin pre-expanded particles, and more particularly to a process for preparing polyolefin resin pre-expanded particles which can be preferably used as, for instance, a raw material for an internal die expansion molded article.
Conventionally, when polyolefin resin pre-expanded particles are prepared, there is well known a process for preparing polyolefin resin expanded particles, comprising dispersing resin particles together with a dispersant into an aqueous disperse medium in a closed container, introducing a volatile blowing agent thereto, then heating the resin particles to at lowest softening temperature of the polyolefin resin and releasing the resin particles into a lower pressure atmosphere than internal pressure of the closed container.
For instance, there is already published a process for preparing polyolefin resin pre-expanded particles having a special crystal structure which shows two melting points in a DSC curve obtained by a differential scanning calorimetry, by using the above process for preparing polyolefin resin expanded particles (see, for instance, Japanese Unexamined Patent Publication No. 176336/1984 and Japanese Unexamined Patent Publication No. 183832/1988). Furthermore, there is already published a method for improving expansion ratio of the above polyolefin resin pre-expanded particles having a special crystal structure with retaining a closed cell structure. In this method, expandability is imparted to the polyolefin resin pre-expanded particles and then, the particles are heated (see Japanese Unexamined Patent Publication No. 23428/1985 and Japanese Unexamined Patent Publication No. 90228/1985).
In Japanese Unexamined Patent Publication No. 23428/1985, there is disclosed a process for preparing pre-expanded particles having higher expansion ratio than that of base non-crosslinked propylene random copolymer pre-expanded particles having a specific range of expansion ratio and cell number and having a crystal structure which shows higher temperature peak on a higher temperature point than a point of intrinsic peak of the base resin in a DSC curve. In this process, expandability is imparted to the non-crosslinked propylene random copolymer pre-expanded particles and then, the particles are heat expanded.
In Japanese Unexamined Patent Publication No. 90228/1985, there is disclosed a method comprising a step of imparting expandability to polypropylene resin pre-expanded particles having the same special crystal structure as that of the pre-expanded particles disclosed in Japanese Unexamined Patent Publication Nos. 176336/1984 and 183832/1988 and having decreasing speed coefficient of internal pressure "k" of at most 0.30, a step of heating the polypropylene resin pre-expanded particles in a closed container to temperature "T" (.degree. C.) represented by the inequality: EQU Tm-65&lt;T&lt;Tm-30
wherein Tm is melting finishing temperature of the base resin, a step of keeping as it is, a step of opening the edge of the container and a step of releasing the polypropylene resin pre-expanded particles into a lower pressure atmosphere than internal pressure of the container.
However, all technologies in these methods relate to a polypropylene resin, in particular, ethylene-propylene random copolymer. Accordingly, technologies using the other polyolefin resin are not disclosed.
As to heat temperature of the pre-expanded particles, as usual, it is described that vapor having vapor pressure of 0.8 to 1.5 kg/cm.sup.2 (G) (116.degree..about.127.degree. C.) or hot air having temperature of at lowest 100.degree. C. is preferably used (see Japanese Unexamined Patent Publication No. 23428/1985). Also, it is described that the pre-expanded particles are heated at temperature "T" (.degree. C.) represented by the inequality: EQU Tm-65&lt;T&lt;Tm-30
wherein Tm is melting finishing temperature of the base resin (see Japanese Unexamined Patent Publication No. 90228/1985). However, although the pre-expanded particles having a special crystal structure are used, it is not disclosed that crystal characteristic of the pre-expanded particles is related to heat temperature of the pre-expanded particles to accomplish the objects.
During heat expansion of the thus polyolefin resin expanded particles, in the case that heat temperature of the expanded particles is too high, open cell ratio of the obtained pre-expanded particles increases. In the case that the thus obtained pre-expanded particles are used for internal die mold, it is forecasted that mechanical strength of the produced molded article remarkably lowers or mutual fusion of the expanded particles is generated to cause short shot during feeding of the expanded particles into a molding machine. These phenomena are mentioned in the above Publications.
Then, the inventors of the present invention have earnestly studied as to heat expansion and carried out the following examination. An ethylene-propylene random copolymer having the above temperature "Tm" of 157.degree. C. was used as a base resin and subjected to the general procedure to give expanded particles. The obtained expanded particles were sufficiently washed with water and an acidic aqueous solution to give expanded particles having the above special crystal structure (expansion ratio: 11.3 times, average cell diameter: 300 .mu.m, decreasing speed coefficient of internal pressure "k": 0.15). The obtained expanded particles having the special crystal structure were heat expanded with vapor having vapor pressure of 1.5 kg/cm.sup.2 (G) (about 126.degree. C.) to give pre-expanded particles (expansion time: 30 seconds). However, although pre-expanded particles having high expansion ratio and a closed cell structure could be certainly prepared, mutual fusion of the expanded particles was generated to cause short shot of the expanded particles during molding.
When pre-expanded particles are prepared by a heat expansion method, as one of the effective means for heightening expansion ratio of the obtained pre-expanded particles, it can be proposed that a resin layer on the surface of expanded particles is softened together with heightening heat temperature and internal pressure of the expanded particles. However, in the case that mutual fusion of the expanded particles is once generated as mentioned above, substantially, heat temperature cannot be heightened any more, so that expansion ratio of the obtained pre-expanded particles cannot be heightened.
On the other hand, in a process for producing a polyolefin resin expansion molded article comprising filling a molding machine with the above pre-expanded particles having the special crystal structure and subjecting the pre-expanded particles to heat fusion with vapor to give an expansion molded article having the desired shape, the amount of attachment on the surface of the pre-expanded particles is known as one of factors exerting an influence upon molding fusion property (see, for instance, Japanese Unexamined Patent Publication No. 57838/1992).
That is, in a process for preparing polyolefin resin expanded particles comprising dispersing polyolefin resin particles into an aqueous disperse medium in a closed container, introducing a volatile blowing agent thereto, then heating the resin particles to at lowest softening temperature of the polyolefin resin and releasing the resin particles into a lower pressure atmosphere than internal pressure of the closed container, inorganic powder which is called dispersant or fusion preventing agent is added to the aqueous disperse medium together with the polyolefin resin particles, the inorganic powder is attached to the surface of the polyolefin resin particles to prevent mutual fusion of the resin particles in the aqueous disperse medium, and this surface attachment is washed and removed during or after preparation of pre-expanded particles to improve molding fusion property.
For instance, in the process described in Japanese Unexamined Patent Publication No. 57838/11992, 3 parts (parts by weight, hereinafter referred to the same) of powdery tricalcium phosphate as a dispersant and 0.12 part of sodium n-paraffinsulfonate are added to 100 parts of ethylene-propylene random copolymer pellet, and a method for washing pre-expanded particles just after preparation is changed, so that the amount of attachment (mainly, tricalcium phosphate) on the surface of the pre-expanded particles is changed. However, in the case that the amount of this attachment is at least 3300 ppm, there is a problem that molding fusion property remarkably lowers, so melt bonding ratio of the obtained molded article becomes 0%. This problem is pointed out in this Publication.
Then, the inventors of the present invention have firstly considered that mutual fusion of expanded particles during heat expansion can be prevented or restrained in accordance with adjustment of the amount of attachment on the surface of the pre-expanded particles. So, by the inventors, there has been carried out heat expansion experiment comprising increasing the amount of attachment on the surface of the base expanded particles under the entirely same condition as in the case that mutual fusion of expanded particles has been generated at vapor pressure of 1.5 kg/cm.sup.2 (G) during the above heat expansion. As a result, it has been found that mutual fusion of expanded particles has not been generated even at vapor pressure of 3 kg/cm.sup.2 (G) (=143.degree. C.).
In addition, by using the other polyolefin resin or composition thereof, such as ethylene-propylene random copolymer, ethylene-.alpha.-olefin copolymer or a resin composition prepared by adding an ethylene ionomer to ethylene-.alpha.-olefin copolymer, which has different melting finishing temperature each other, the same experiment as mentioned above has been carried out. As a result, it has been found that at lower temperature than T.sub.L -30 (.degree. C.) (T.sub.L : lower melting point of the two melting points of base pre-expanded particles), expansion ratio by heat expansion has been improved only a little. Also, it has been found that at higher temperature than T.sub.V (.degree. C.) (T.sub.V : saddle temperature between two peaks showing two respective melting points), mutual fusion of expanded particles has been generated having no relation to melting finishing temperature of base resin (composition) or no relation to the amount of attachment on the surface of base expanded particles, or open cell ratio of the obtained pre-expanded particles has become at least 20% by heat expansion. Accordingly, it has been found that retention of good moldability has been difficult.
Furthermore, at the temperature range from T.sub.L -30 (.degree. C.) to T.sub.V (.degree. C.), there has been examined the relation between the amount of attachment on the surface of the expanded particles and t (.degree. C.) (t: heat expansion temperature at which mutual fusion of expanded particles has been generated). As a result, as to all kinds of polyolefin resin expanded particles, surprisingly, it has been found that there has been the linear relation between the upper limit of the temperature represented by t-T.sub.V (.degree. C.) (at the same temperature as this upper limit or higher temperature than this upper limit, mutual fusion of expanded particles has been generated) and the amount of a fusion preventing agent being attached to the surface of the expanded particles, so that the present invention has been completed.
The present invention has been accomplished in consideration of the above knowledge.
An object of the present invention is to provide a process for preparing polyolefin resin pre-expanded particles having high expansion ratio with retaining closeness of cell and without generation of mutual fusion of expanded particles in accordance with a heat expansion method by using the above polyolefin resin expanded particles having a special crystal structure.
This and other objects of the present invention will become apparent from the description hereinafter.